Semi-finished products and components for motor vehicles must satisfy various requirements depending on their location and purpose of use in a motor vehicle, particularly in terms of their mechanical properties and corrosion properties.
In the case of interior door panels, for example, the mechanical properties are mainly determined by stiffness, which particularly depends on the shaping of said parts. By comparison, strength is less influential, though the materials used must not be too soft either. At the same time, good formability is very important since the parts and semi-finished products generally undergo complex forming processes, for example for producing interior door panels. This applies particularly to components that are prepared in a one-part sheet metal shell construction, such as for example a sheet metal interior door with integral window frame area. By eliminating joining operations, such components offer cost advantages over an attached profile solution for the window frame.
It would be particularly advantageous if a corresponding semi-finished product or component could be formed from an aluminium alloy on a tool for steel components, since aluminium or steel components could then be produced as needed on the same tool, thereby reducing or eliminating investment and operating costs for an additional tool.
For the reasons mentioned above, there is great interest in the automobile industry for medium-strength aluminium alloys with very good formability, particularly if they have better formability than the standard alloy AA (Aluminium Association) 5005 (AlMg1), for example.
Besides mechanical properties, corrosion resistance is also a major consideration for motor vehicles since motor vehicle components such as interior door panels are exposed to splashed, condensation or perspiration water. It is therefore desirable for the motor vehicle components to have good resistance to various corrosive attacks, particularly intercrystalline corrosion and filiform corrosion.
Filiform corrosion is the term used for a corrosion type that occurs on coated components and has a thread-like pattern. Filiform corrosion occurs in high humidity in the presence of chloride ions.
Attempts have been made in the past to produce semi-finished products and components for motor vehicles from the alloy AA 8006 (AlFe1.5Mn0.5). Although semi-finished products having sufficient strength and very good formability can be produced with this alloy, the corresponding components were highly susceptible to filiform corrosion after painting, so that the alloy AA 8006 is not suitable for coated, particularly painted components such as interior door panels.
Curable AA 6xxx alloys are very strong and have good resistance to intercrystalline and filiform corrosion, but they are considerably more difficult to form than AA 8006, and therefore not well suitable for producing complex parts such as for example interior door panels. Moreover, production of semi-finished products and components from an AA 6xxx alloy is very complex and expensive, since they must undergo continuous annealing as a special process step.
AA 5xxx alloys with high magnesium content combine good strength properties with very good formability. However, this formability is not equal to those of steel solutions, which leads to restrictions in the design of the components. Moreover, said alloys are prone to intercrystalline corrosion. Steel materials have high formability, but their weight is unfavourable for the same stiffness, and they are also susceptible to corrosion.